(1) Field of the Invention
This invention generally relates to sonar systems and more specifically to sonar systems particularly adapted for identifying the location of an underwater object.
(2) Description of the Prior Art
Conventional passive sonar systems detect acoustic signals emanating from an "underwater object;" that is, any device that moves through the water while emitting acoustic signals that sonar can detect. Torpedoes and submarines are examples of such "underwater objects."
Sonar systems employ a variety of technologies to detect and locate such underwater objects. Generally, however, acoustic signals from a specific object define a complex wavefronts and pass a towed horizontal array or a stationary vertical array of hydrophones at different times. U.S. Pat. No. 4,965,732 to Roy et al. for Methods and Arrangements for Signal Reception and Parameter Estimation and U.S. Pat. No. 5,216,640 to the same inventors as in this application for an Inverse Beamforming Sonar System and Method are examples of such sonar systems. Some sonar systems, such as disclosed in our prior patent, implement a beamforming process to correlate the signals received at each hydrophone and to decompose these signals into a signal representing the bearing to a particular underwater object.
Many other prior art beamforming processes typically obtain a bearing by time delaying the received signal from each hydrophone. However, the hardware for implementing this process is bulky and heavy. Shipboard constraints on available space and load weight often limit or even preclude on board systems using this technology. Moreover, this apparatus can only form a limited number of beams, and this limit can establish a maximum number of underwater objects that can be monitored simultaneously. Even when multiple underwater objects can be detected, confidence in that detection only exists if the individual underwater objects are widely spaced in azimuth. Otherwise it is possible that low intensity signals located between the individual beams will be lost. Many conventional sonar systems also use fixed threshold levels to avoid erroneous signal declarations from random noise emanating from the ocean, so low intensity signals from underwater objects also may not be detected because they do not reach the threshold.
U.S. Pat. No. 5,216,640 discloses a sonar system that measures plane wave energy at multiple points using a horizontal array of plane wave energy receptors such as hydrophones. An inverse beamforming processor generates output beam levels for analysis in a spectrum normalizer to yield spatially and spectrally normalized output beam levels. An eight nearest-neighbor peak-picker analyzes the normalized output beam levels to identify plane wave peaks that a three-dimensional M of N tracker uses to identify peaks within a specified limit of frequency and angle change over time. This system enables the detection, with confidence, of closely angularly spaced underwater objects even where signal-to-noise ratios are low. However, this system only provides bearing angle and frequency measurements (i.e., one-dimensional location of the underwater object relative to the sensing system such as the hydrophone array). This system cannot provide range and depth information that is necessary to locate the object in three-dimensional underwater space. Rather, independent systems, such as those using stationary vertical hydrophone arrays, supplement systems using inverse beamforming to obtain bearing information.